1. Field of the Present Description
The technical field is vibration attenuators for rotor hubs.
2. Description of Related Art
Rotary-wing aircraft, such as helicopters and tiltrotors, have at least one rotor for providing lift and propulsion forces, and these rotors have at least two airfoil blades connected to a rotatable hub. The blades cause vibrations that are a function of the rotational speed of the rotor, and aircraft designers have difficulty accurately predicting the exact vibration modes that a particular rotor configuration will encounter. The vibrations can be transmitted through the rotor mast, through associated powertrain components, and into the airframe of the aircraft. The vibrations can reduce the life of affected components and cause undesirable vibrations for passengers. Various types of vibration attenuation systems have been developed to reduce or eliminate these vibrations. The prior art includes airframe-mounted vibration attenuators and at least one mast-mounted system.
Active systems in the prior art act at a specific point on the airframe to reduce vibrations, and this can result in amplified vibrations in other locations on the airframe. However, a passive mast-mounted rotating balancer for vibration reduction was disclosed in U.S. Pat. No. 3,219,120 and in an American Helicopter Society paper entitled, “UREKA-A Vibration Balancing Device for Helicopters” (Jan. 1966, Vol. 11, No. 1). The UREKA (Universal Rotor Excitation Kinematic Absorber) device uses heavy rollers which revolve in a circular steel track to create an oscillatory force to minimize vibration. The rollers are free to rotate and position themselves relative to the position of the rotor, and the rollers will automatically achieve the correct position to minimize vibration if the mast attachment point possesses specific dynamic characteristics. However, the UREKA system only prevents an imbalance of the rotor, and does not oppose other rotor-induced vibrations. The dynamic characteristics necessary for proper operation are basically those of a supercritical shaft. If the mast attachment point does not possess these characteristics, then the UREKA device will amplify vibration rather that attenuate it. In addition, since the position of the rollers is governed by the motion of the mast attachment point, the device is susceptible to gusts and other transients which may disturb the roller position, creating a vibration transient.
For application to tiltrotors, where large changes in gross weight and rotor rotational speed are present, the UREKA device may not function properly, as the dynamic characteristics of the mast attachment point would vary considerably. The V-22 tiltrotor, for example, has dynamic characteristics which prevent the use of the UREKA design. One method developed for the 3-blade V-22 aircraft includes passive pendulums for controlling vibrations.
Although great strides have been made in the art of vibration attenuators for rotor hubs, significant shortcomings remain.